The size of a PMDC motor is a very critical factor in some applications. There is a desire for a smaller motor having equal or increased performance. In other words, there is a desire for a motor having a higher power density. U.S. published patent application No. US-2010-0231072-A1 discloses a PMDC motor having increased power density compared with a traditional flat sided motor. The motor comprises a housing made of steel and having four side portions and four connecting portions. Adjacent side portions are connected by a corresponding connecting portion. Magnets are fixed to the connecting portions. The radial cross section of each side portion is like a convex curved line, which is curved outwardly relative to a straight line passing through the two ends of the convex curve line. Compared to a traditional motor with two flat sides, this motor has increased power density and lower noise according to finite element analysis (FEA) test. However, the housing with curved side portions makes it not easy to install the motor.
Therefore, there is a desire for an improved motor that is easy to install and has equivalent or higher power density.